Overview of Java polymorphism: (Recommended: java video tutorial)
Polymorphism is an important feature of object-oriented programming languages, which allows basic The pointer or reference of the class points to the object of the derived class, and the dynamic binding of the method is implemented during specific access. Java The implementation of dynamic binding of method calls mainly relies on the method table, but the implementation through class reference calls (invokevitual) and interface reference calls (invokeinterface) is different.
Three conditions are required to implement polymorphism in java:
1. There needs to be an inheritance relationship.
2. There needs to be method rewriting.
3. There needs to be a reference from the parent class pointing to the subclass object.
The general process of class reference calling is: the Java compiler compiles the Java source code into a class file. During the compilation process, the called symbol reference is written into the class file according to the static type.
During execution, the JVM finds the symbolic reference of the calling method according to the class file, then finds the offset in the static type method table, and then determines the actual type of the object based on this pointer, using the actual type method table , the offset is the same as the offset of the method table in the static type. If the method is found in the method table of the actual type, it is called directly. Otherwise, it is considered that the method of the parent class has not been overridden. Search from bottom to top according to inheritance relationship.
As can be seen from the above figure, when the program is running and a certain class is required, the class loading subsystem will load the corresponding class file into the JVM. And internally establish the type information of the class (this type information is actually a data structure stored in the JVM by the class file), including all the information defined by the java class, including method code, class variables, member variables, and the content of this blog post. Key discussion methods table. This type information is stored in the method area.
Note that the type information in this method area is different from the class object stored in the heap. In the method area, the type information of this class has only a unique instance (so it is a memory area shared by each thread), and there can be multiple objects of this class in the heap. The type information in the method area can be accessed through the class object in the heap. Just like the Java reflection mechanism, all information of the class can be accessed through the class object.
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